This section is intended to introduce various aspects of the art, which may be associated with exemplary embodiments of the present invention. This discussion is believed to assist in providing a framework to facilitate a better understanding of particular aspects of the present invention. Accordingly, it should be understood that this section should be read in this light, and not necessarily as admissions of prior art.
Arctic offshore regions are continuing to receive more interest by oil and gas development companies. However, due to the presence of ice floes and icebergs, conducting hydrocarbon extraction related operations, such as, but not limited to, hydrocarbon production and drilling, in offshore arctic locations is difficult.
A conventional offshore drilling system is depicted in FIG. 1. As depicted, a vessel 101 floats in the water 103. The position of both the vessel 101 and a wellhead 105, which is positioned on the seafloor 107, are fixed relative to each other using thrusters or other known techniques. For a drilling vessel, each installation typically includes a single riser 109 used to connect the wellhead 105 to the vessel 101 and pass drilling materials such as, but not limited to, drilling fluid, drill bit and string, casings, and cement. As appreciated by those skilled in the art, wellhead 105 may be equipped with additional hardware, such as, but not limited to, a blowout preventer or a lower marine riser package.
When drilling in offshore arctic locations, it may be required to disconnect from the wellhead 105 due to intrusions of unmanageable ice 111 flowing into the watch circle, or area surrounding the vessel 101. Based on the vertical configuration of the riser 109, the vessel 101 must remain relatively stationary over the wellhead 105 in order to protect the riser 109 and its connection to the wellhead 105. There is some horizontal tolerance 113 in the vessel's position, though it is typically limited by some amount, often less than 5% of the water depth (or riser length), in order to prevent damage to the riser 109. Because of the limited horizontal tolerance of the vertical riser, ice floes (particularly in shallow water) pose a significant risk to riser integrity. Therefore, small icebergs or other dangerous ice features that may cause damage to the rig or well must be detected early enough to disconnect the riser or allow for the ice to otherwise be mitigated. In addition to impending ice 111, the vessel 101 may drift off of its fixed position due to a variety of conditions, such as, but not limited to, wind, waves, current or drive off due to thruster malfunction.
Though drift-off and drive-off are rare, such conditions are not acceptable as an operational norm as they require emergency measures to disconnect the riser 109. It is therefore desirable to limit the number of riser disconnections.
In some Arctic environments, such as those with significant icebergs or pack ice, potential ice features exceeding any practical resistance may frequently occur. It is difficult to accurately forecast multi-day ice drift patterns. As a result, the state of the art strategy is to either schedule drilling when there is no threat of significant ice or to actively manage the ice through iceberg towing or lead icebreakers in pack ice. However, there are potential locations, such as, but not limited to, those near the toe of a glacier or an ice shelf, where the threat of significant ice features is nearly year-around and there is a significant probability that the ice is either too large to be managed or escapes active ice management. For example, the casing/cementing of a wellbore may take several days and it is unacceptable to disconnect the riser during such operations. Therefore, significant risks are associated with drilling in icy regions. In such locations an alternative strategy is needed to enable drilling and related operations without increased occurrence of emergency disconnect.
Thus, there is a need for improvement in this field.